Heat treatment of steel for resistance to creep



Patented Mar. .23, 1 937 UNITED STATES PATENT OFFICE HEAT TREATMENT OF STEEL FOR RESIST- ANCE TO CREEP 5 George H. Wright, Ballston Spa, N. Y., assignor to General'Electric Company, a corporation of New York No Drawing.

Application Septcrnben28, 1934, Serial No. 745,853

' 8 Claims. (t31l .14821.5)

ber of pounds per square inch which, applied to a.

10 material such as steel, will cause it to stretch exactly 1% in 100,000 hours.

In the construction of ste'am turbines, .it is desirable to eliminate or prevent, as far as it is possible to do so, any creep in the shafting or in the fastening means, such as bolts or the like,

as tensile strength, elastic limit, elongation and reduction of area but which develop a minimum amount of creep. Prior to the present invention, it has'been substanti'ally impossible to obtain consistently steel having a creep value ,as high as 20,000 pounds per square inch at temperatures of about 450 C..or

higher. It is one of the objects of the present in- ,30 vention to provide steel which not only has a high creep value at temperatures of about 450 C. but whichhas desirable physical characteristics as well.

'In carrying out my invention, steel, which may be a hypo-eutectoid forging, is heated to a temp'erature above the critical range of the steel and belowthe point of incipient fusion, for example, about 175 C. or more above the upper critical point and maintained at that temperature for- 40 about eight hours, :or long ,enough to eliminate the appearance of'any dendrites or banded structure in the steel after cooling.- 'I'he forging may be brought up to the desired elevated temperature by placing'it in'a furnace heated at about 300 C.

4.5 and then slowly increasing the temperature until the desired temperature above the criticalrang'e.

is reached. The time required to heat the forge ing to this temperature'will vary, depending upon the size and character of the forging, but will usually be about one hour per inch of thickness. After the desired temperature is attained, the

i steel is held at that temperature for about eight hours or long enough to promote diffusionof carhon and to thereby substantially eliminate the appearance of any banded or dendritic structure after cooling. After the heat treatment the forging may be oil quenched or air cooled, depend-,

ing uponthe size of the forging. For example,

forgings of large cross section should in general be oil quenched. In cooling the steel in oil or air, I 5

have found it desirable to cool to a temperature of about C. and then to reheat to a temperature several hundred degrees abpve room temperature to thereby relieve quenching strains.

After. reheating, it has'been found desirable to 10 again hold the forging in the furnace for a period of time equal to about one hour for each inch of thickness of the forging.

The following is given as an illustration only of the invention, 7 e

, A forging containing about 0.37% carbon and 1.75% nickel, 0.83% chromium, 0.3% molybdenum with the remainder, iron except for incidental impurities, was heated at about 950 C. for eight hours, air cooled, reheated at 650 C. and then, 20 cooled in the furnace. Asa result of this heat treatment, this forging hasa creep value at 450 C. of 33,800 pounds per square inch, and in addition, at room temperature, a tensile strength of 121,000 pounds per square inch, an elastic 25 limit of 80,000 pounds per square inch, an elongation in 2 of 20% and a reduction of area. of 58%. The resulting structure obtained by the heat treatment was coarse grained with a structure of uniform appearance.

Steel having the identical composition disclosed above when heated-at a temperature of about 825 0., oil quenched and then reheated ance to creep as well as satisfactory physical qualities, Good results also may be obtained by employing an elevated temperature somewhat lower than 950 C. for example about 825 C. provided the forging isquenched or quickly cooled from that temperature. The temperature. em- -50 ployed for the draw or reheating may vary considerably and will depend in general 'upon the physical properties desired in the finished product. As arule', the temperature of 600 C. to 650 C. will provide satisfactory results. 55

While the heat treatment here and before set forth provides a steel structure substantially free from bands and having a high creep value, the most satisfactory results will be obtained if the steel to be given the heat treatment is substantially free from dendrites prior to the heat treatment. Under such conditions a steel composition, such as above disclosed, after the heat treatment may have creep values up to about 40,000 pounds per square inch at 450 C.

What I claim as new and desire to secure by Letters Patent of the United States is: I

l 1. The method for'increasing the resistance of forged hypo-eutectoid steel to creep which comprises heating the steel at temperature at least about 0. above its critical range but below the point of incipientfusion for several hours, then quenching the steel.

2. The method for increasing the resistance of forged hypo-eutectoid steel to creep which comprises heating the steel at a temperature at least about 175 C. above its critical range but below the point of incipient fusion for about eight or more hours and air cooling the steel.

3. The method. for increasing the resistance of forged hypo-eutectoid steel to; creep which comprises heating the steel at a temperature at least about 175 0. above its critical range but below the point of incipient fusion for a period of time long enough 'to eliminate any banded structure after cooling, quenching and then reheating the steel at a lower temperature to thereby remove quenching strains.

4. The method for increasing the resistance of dendrites at a temperature at least about 175 C. above the critical range of the steel but below its point of incipient fusion for a period of time/ equal to about one hour per inch of thickness of the steel.

5. The method for increasing the resistance to creep of forgedsteel consisting substantially of 0.37% carbon, 1.75% nickel, 0.83% chromium, 0.37% molybdenum, and the remainder iron,

. which comprises heating the steel to about 950 C. for at least eight hours, quenching and reheating at about 600 C. to 650 C. thereby to relieve quenching strain.

6. The method for increasing the creep resistance of a forged hypo-euctectoid nickel, chrome, molybdenum steel which comprises heating the steel at a temperature of at least 950 C. for at least eight hours,- quenching and reheating the steel at about 600 to 650 C.

7.,A device, a portion of which is subject to. continued stress at elevated temperatures, said. portion comprising a forged hypo-eutectoid steel which'has been heated for several hours at a temperature in the range between about 175 C. above the critical range of the steel and below its point of incipient fusion. I

8. A tension member subject to continued stress at elevated temperatures, said member consisting of a forged hypo-eutectoid nickel, chrome, molybdenum steel which has been heated for at least eight hours at a temperature at least 175 C. above the critical range of the steel but below its point of incipient fusion.

. GEORGE r1. WRIGHT. 

